Nanomachining, by definition, involves mechanically removing nanometer-scaled volumes of material from, for example, a photolithography mask, a semiconductor substrate/wafer, or any surface on which scanning probe microscopy (SPM) can be performed. For the purposes of this discussion, “substrate” will refer to any object upon which nanomachining may be performed.
Examples of photolithography masks include: standard photomasks (193 nm wavelength, with or without immersion), next generation lithography mask (imprint, directed self-assembly, etc.), extreme ultraviolet lithography photomasks (EUV or EUVL), and any other viable or useful mask technology. Examples of other surfaces which are considered substrates are membranes, pellicle films, micro-electronic/nano-electronic mechanical systems MEMS/NEMS. Use of the terms, “mask”, or “substrate” in the present disclosure include the above examples, although it will be appreciated by one skilled in the art that other photomasks or surfaces may also be applicable.
Nanomachining in the related art may be performed by applying forces to a surface of a substrate with a tip (e.g., a diamond cutting bit) that is positioned on a cantilever arm of an atomic force microscope (AFM). More specifically, the tip may first be inserted into the surface of the substrate, and then the tip may be dragged through the substrate in a plane that is parallel to the surface (i.e., the xy-plane). This results in displacement and/or removal of material from the substrate as the tip is dragged along.
As a result of this nanomachining, debris (which includes anything foreign to the substrate surface) is generated on the substrate. More specifically, small particles may form during the nanomachining process as material is removed from the substrate. These particles, in some instances, remain on the substrate once the nanomachining process is complete. Such particles are often found, for example, in trenches and/or cavities present on the substrate.
In order to remove debris, particles or anything foreign to the substrate, particularly in high-aspect photolithography mask structures and electronic circuitry; wet cleaning techniques have been used. More specifically, the use of chemicals in a liquid state and/or agitation of the overall mask or circuitry may be employed. However, both chemical methods and agitation methods such as, for example, megasonic agitation, can adversely alter or destroy both high-aspect ratio structures and mask optical proximity correction features (i.e., features that are generally so small that these features do not image, but rather form diffraction patterns that are used beneficially by mask designers to form patterns).
In order to better understand why high-aspect shapes and structures are particularly susceptible to being destroyed by chemicals and agitation; one has to recall that such shapes and structures, by definition, include large amounts of surface area and are therefore very thermodynamically unstable. As such, these shapes and structures are highly susceptible to delamination and/or other forms of destruction when chemical and/or mechanical energy is applied.
It is important to note that in imprint lithography and EUV (or EUVL) that use of a pellicle to keep particles off the lithographic surface being copied is currently not feasible. Technologies that cannot use pellicles are generally more susceptible to failure by particle contamination which blocks the ability to transfer the pattern to the wafer. Pellicles are in development for EUV masks, but as prior experience with DUV pellicle masks indicates, the use of a pellicle only mitigates (but does not entirely prevent) critical particle and other contaminates from falling on the surface and any subsequent exposure to the high-energy photons will tend to fix these particles to the mask surface with a greater degree of adhesion. In addition, these technologies may be implemented with smaller feature sizes (1 to 300 nm), making them more susceptible to damage during standard wet clean practices which may typically be used. In the specific case of EUV or EUVL, the technology may require the substrate be in a vacuum environment during use and likely during storage awaiting use. In order to use standard wet clean technologies, this vacuum would have to be broken which could easily lead to further particle contamination.
Other currently available methods for removing debris from a substrate make use of cryogenic cleaning systems and techniques. For example, the substrate containing the high-aspect shapes and/or structures may be effectively “sandblasted” using carbon dioxide particles instead of sand.
However, even cryogenic cleaning systems and processes in the related art are also known to adversely alter or destroy high-aspect features. In addition, cryogenic cleaning processes affect a relatively large area of a substrate (e.g., treated areas may be approximately 10 millimeters across or more in order to clean debris with dimensions on the order of nanometers). As a result, areas of the substrate that may not need to have debris removed therefrom are nonetheless exposed to the cryogenic cleaning process and to the potential structure-destroying energies associated therewith. It is noted that there are numerous physical differences between nano and micro regimes, for the purposes here, the focus will be on the differences related to nanoparticle cleaning processes. There are many similarities between nano and macro scale cleaning processes, but there are also many critical differences. For the purposes of this disclosure, the common definition of the nanoscale is of use: this defines a size range of 1 to 100 nm. This is a generalized range since many of processes reviewed here may occur below this range (into atomic scales) and be able to affect particles larger than this range (into the micro regime).
Some physical differences between macro and nano particle cleaning processes include transport related properties including: surface area, mean free path, thermal, and field-effects. The first two in this list are more relevant to the thermo-mechanical-chemical behavior of particles while the last one is more concerned with particle interactions with electromagnetic fields. Thermal transport phenomenon intersects both of these regimes in that it is also the thermo-mechanical physical chemistry around particles and the interaction of particles with electromagnetic fields in the infrared wavelength regime. To functionally demonstrate some of these differences, a thought experiment example of a nanoparticle trapped at the bottom of a high aspect line and space structure (70 nm deep and 40 nm wide AR=1.75) is posited. In order to clean this particle with macroscale processes, the energy required to remove the particle is approximately the same as the energy required to damage features or patterns on the substrate, thereby making it impossible to clean the high aspect line and space structure without damage. For macro-scale cleaning processes (Aqueous, Surfactant, Sonic Agitation, etc.), at the energy level where the nanoparticle is removed, the surrounding feature or pattern is also damaged. If one has the technical capability to manipulate nano-sharp (or nanoscale) structures accurately within nano-distances to the nanoparticle, then one may apply the energy to clean the nanoparticle to the nanoparticle only. For nanoscale cleaning processes, the energy required to remove the nanoparticle is applied only to the nanoparticle and not the surrounding features or patterns on the substrate.
First, looking at the surface area properties of particles, there are mathematical scaling differences which are obvious as a theoretical particle (modelled here as a perfect sphere) approaches the nanoscale regime. The bulk properties of materials are gauged with the volume of materials while the surface is gauged by the external area. For a hypothetical particle, its volume decreases inversely by the cube (3rd power) while the surface area decreases by the square with respect to the particle's diameter. This difference means that material properties which dominate the behavior of a particle at macro, and even micro, scale diameters become negligible into the nano regime (and smaller). Examples of these properties include mass and inertial properties of the particle, which is a critical consideration for some cleaning techniques such as sonic agitation or laser shock.
The next transport property examined here is the mean free path. For macro to micro regimes, fluids (in both liquid, gaseous, and mixed states) can be accurately modelled in their behavior as continuum flow. When considering surfaces, such as the surface of an AFM tip and a nanoparticle, that are separated by gaps on the nanoscale or smaller, these fluids can't be considered continuum. This means that fluids do not move according to classical flow models, but can be more accurately related to the ballistic atomic motion of a rarefied gas or even a vacuum. For an average atom or molecule (approximately 0.3 nm in diameter) in a gas at standard temperature and pressure, the calculated mean free path (i.e., distance in which a molecule will travel in a straight line before it will on average impact another atom or molecule) is approximately 94 nm, which is a large distance for an AFM scanning probe. Since fluids are much denser than gasses, they will have much smaller mean free paths, but it must be noted that the mean free path for any fluid can't be less than the atom or molecule's diameter. If we compare the assumed atom or molecule diameter of 0.3 nm given above to the typical tip to surface mean separation distance during non-contact scanning mode which can be as small as 1 nm, thus except for the most dense fluids, the fluid environment between an AFM tip apex and the surface being scanned will behave in a range of fluid properties from rarefied gas to near-vacuum. The observations in the prior review are crucial to demonstrating that thermo-fluid processes behave in fundamentally different ways when scaled from the macro to nano scale. This affects the mechanisms and kinetics of various process aspects such as chemical reactions, removal of products such as loose particles to the environment, charging or charge neutralization, and the transport of heat or thermal energy.
The known thermal transport differences from macro and nano to sub-nano scales has been found by studies using scanning thermal probe microscopy. One early difference seen is that the transport rate of thermal energy can be an order of magnitude less across nanoscale distances than the macro scale. This is how scanning thermal probe microscopy can work with a nano probe heated to a temperature difference of sometimes hundreds of degrees with respect to a surface it is scanning in non-contact mode with tip to surface separations as small as the nano or Angstrom scale. The reasons for this lower thermal transport are implied in the prior section about mean free path in fluids. One form of thermal transport, however, is enhanced which is blackbody radiation. It has been experimentally shown that the Plank limit for blackbody spectral radiance at a given temperature can be exceeded at nanoscale distances. Thus, not only does the magnitude of thermal transport decrease, but the primary type of transport, from conduction/convection to blackbody which is in keeping with the rarefied to vacuum fluid behavior, changes.
Differences in the interactions of fields (an electromagnetic field is the primary intended example here due to its longer wavelengths compared to other possible examples), for the purposes in this discussion, could be further sub-classified as wavelength related and other quantum effects (in particular tunneling). At nanoscales, the behavior of electromagnetic fields between a source (envisioned here as the apex of an AFM tip whether as the primary source or as a modification of a relatively far field source) and a surface will not be subject to wavelength dependent diffraction limitations to resolution that far field sources will experience. This behavior, commonly referred to as the near-field optics, has been used with great success in scanning probe technologies such as near field scanning optical microscopy (NSOM). Beyond applications in metrology, the near field behavior can affect the electromagnetic interaction of all nanoscale sized objects spaced nano-distances from each other. The next near-field behavior mentioned is quantum tunneling where a particle, in particular an electron, can be transported across a barrier it could not classically penetrate. This phenomenon allows for energy transport by a means not seen at macro scales, and is used in scanning tunneling microscopy (STM) and some solid-state electronic devices. Finally, there are more esoteric quantum effects often seen with (but not limited to) electromagnetic fields at nanoscales, such as proximity excitation and sensing of plasmonic resonances, however, it will be appreciated by one skilled in the art that the current discussion gives a sufficient demonstration of the fundamental differences between macro and nano-scale physical processes.
In the following, the term “surface energy” may be used to refer to the thermodynamic properties of surfaces which are available to perform work (in this case, the work of adhesion of debris to the surfaces of the substrate and the tip respectively). One way to classically calculate this is the Gibb's free energy which is given as:G(p,T)=U+pV−TS where:
U=Internal Energy;
p=Pressure;
V=Volume;
T=Temperature; and
S=Entropy.
Since the current practice does not vary pressure, volume, and temperature (although this does not need to be the case since these parameters could equally be manipulated to get the desired effects as well) they will not be discussed in detail. Thus, the only terms being manipulated in the equation above will be internal energy and entropy as driving mechanisms in the methods discussed below. Entropy, since it is intended that the probe tip surface will be cleaner (i.e., no debris or unintended surface contaminates) than the substrate being cleaned is naturally a thermodynamic driving mechanism to preferentially contaminate the tip surface over the substrate (and then subsequently, contaminate the cleaner pallet of soft material). The internal energy is manipulated between the pallet, tip, debris, and substrate surfaces by the thermophysical properties characterized by their respective surface energies. One way to relate the differential surface energy to the Gibbs free energy is to look at theoretical developments for the creep properties of engineering materials at high temperatures (i.e., a significant fraction of their melting point temperature) for a cylinder of radius r, and length l, under uniaxial tension P:dG=−P*dl+γ*dA where
γ=Surface energy density [J/m2]; and
A=Surface area [m2].
The observation that the stress and extrinsic surface energy of an object are factors in its Gibbs free energy induces one to believe these factors (in addition to the surface energy density γ) could also be manipulated to perform reversible preferential adhesion of the debris to the tip (with respect to the substrate) and then subsequently the soft pallet. Means to do this include applied stress (whether externally or internally applied) and temperature. It should be noted that it is intended that the driving process will always result in a series of surface interactions with a net ΔG<0 in order to provide a differential surface energy gradient to preferentially decontaminate the substrate and subsequently preferentially contaminate the soft pallet. This could be considered analogous to a ball preferentially rolling down an incline to a lower energy state (except that, here, the incline in thermodynamic surface energy also includes the overall disorder in the whole system or entropy). FIG. 6 shows one possible set of surface interactions where the method described here could provide a down-hill thermodynamic Gibbs free energy gradient to selectively remove a contaminate and selectively deposit it on a soft patch. This sequence is one of the theoretical mechanisms thought to be responsible for the current practice aspects using low surface energy fluorocarbon materials with medium to low surface energy tip materials such as diamond.